1. Field of the Invention
The present invention relates to a motor driving system that employs either an estimated value of a magnetic pole position or a detected value of a magnetic pole position corresponding to a normal or abnormal condition in sensorless control of the motor driving system. The invention also relates to an electric vehicle provided with such a motor driving system.
2. Description of the Related Art
In order to reduce a size and a cost of a driving system for a synchronous motor, a drive control method so-called sensorless control method has been proposed that does not employ a magnetic pole position detecting device to detect the magnetic pole position of a rotor.
The sensorless control, as is well known, executes estimation operation of the magnetic pole position of a rotor from the information on the terminal voltage and the armature current of the motor, and performs torque control and speed control of the motor by controlling current based on the estimated magnetic pole position.
Known traditional technologies for a motor driving system employing the sensorless control method are disclosed in Japanese Unexamined Patent Application Publication No. 2001-112282 and Japanese Unexamined Patent Application Publication No. 2007-209105, for example.
Japanese Unexamined Patent Application Publication No. 2001-112282 (paragraphs [0007] and [0011], and FIG. 1, in particular) discloses a motor control device for driving a motor, comprising a magnetic pole position detecting sensor and a magnetic pole position estimating device that employs sensorless control. In normal operating conditions, the motor is driven based on the position value detected by the magnetic pole position detecting sensor, and if any failure has occurred in the magnetic pole position detecting sensor, the control is changed over to execution based on the position value estimated by the magnetic pole position estimating device.
Japanese Unexamined Patent Application Publication No. 2007-209105 (Paragraphs [0013] through [0020], and FIGS. 1 and 2, in particular) discloses an electric vehicle driving device that compares a detected value of a magnetic pole position from a magnetic pole position detector and an estimated value of the magnetic pole position by a sensorless controller. If the difference between the detected value and the estimated value exceeds a predetermined value, some failure in the magnetic pole position detector is determined.
In the conventional technology disclosed in Japanese Unexamined Patent Application Publication No. 2001-112282, although the information on the magnetic pole position is doubled improving reliability, the provision of the magnetic pole position detector, an abnormality detecting device, and the magnetic pole position estimating device enlarges the system and raises the costs.
In the conventional technology disclosed in Japanese Unexamined Patent Application Publication No. 2007-209105, the abnormality of the magnetic pole position detector is determined based on the assumption that the magnetic pole position value estimated by the sensorless controller is correct. As a consequence, if the sensorless control itself becomes unstable, the accuracy of magnetic pole position estimation deteriorates, which leads to erroneous determination of abnormality of the magnetic pole position detector, and even run-away of the motor may occur.
A motor driving system having a sensorless control algorithm and without a magnetic pole position detector should be smaller and cheaper than a traditional driving system using a magnetic pole position detector. Such a motor driving system without a magnetic pole position detector has an additional advantage that the failure rate of the overall system is reduced because of elimination of problems due to vibration, heating, or noise.
If the reliability of the sensorless control is not guaranteed, however, a driving system depending on the sensorless control cannot be employed without hesitation for such an application as electric vehicles that require absolute safety.